The reliability of microscopic conductors deposited on integrated circuits formed on a wafer, can be evaluated by forming a long serpentine resistance structure or assay conductor on the same wafer and measuring the resistance between the ends of the serpentine assay conductor. The resistance between the ends of the serpentine conductor determines the "measured length" of the serpentine conductor. This measured length is then compared to the design length of the serpentine conductor which is known from the design layout of the serpentine conductor, to determine discrepancies that may indicate defects in the serpentine conductor. A similar method is to compare the resistance between the ends of the serpentine conductor and the level of resistance that would be expected in the absence of defects, of an ideal serpentine conductor. Both methods are equivalent and can be used interchangeably.
In some integrated circuits, the microscopic conductors of the circuit pass along steps connecting regions of different heights. There is a considerable possibility of defects in a conductor along the steps where the conductor extends between the regions of different heights. Also, the conductor may have different widths at the different heights, due to masks through which light is passed to define the conductors, being out of focus. A higher than expected resistance for a serpentine assay conductor extending along the multiple steps, could be due to the conductor having a smaller than expected width, which does not indicate unreliability. A testing method and apparatus which enabled assessment of the causes of a discrepancy between the ideal and the measured resistance of the serpentine assay conductor, would be of considerable value.